In semiconductor manufacturing factories or liquid crystal manufacturing factories, pure nitrogen obtained by vaporizing liquefied nitrogen has been hitherto utilized in a large amount for cleaning or purging use in respective steps. In recent years, however, the use of cheap CODA is increasing in place of pure nitrogen.
In a plant for producing CDA, in answer to such demand, there have been usually adopted adsorption units of the TSA (Temperature Swing Adsorption) system (Heating regeneration unit) and PSA (Pressure Swing Adsorption) system (Pressure reduction regeneration unit), which are of the two-column switching system using molecular sieve or silica gel. By improving the efficiency and operability or attempting improvements in cost, furthermore, there has been proposed various compressed air producing methods and producing plants.
As shown in FIG. 6, for example, there is disclosed a method and plant for supplying highly pure CDA and dry air that has been hitherto used at the same time, with good efficiency and in stability. Namely, a compression step of compressing feed air by an air compressor 101, a prepurification step of removing moisture in the feed air by a prepurifier 103, a catalytic purification step of converting hydrogen and carbon monoxide in the feed air to moisture and carbon dioxide by a catalytic purifier 105 and an adsorptive purification step of removing moisture and carbon dioxide by an adsorptive purifier 106 are performed in this order, whereby highly pure dry air can be obtained and dry air that has passed through said prepurification step can be collected as a product at any time (see: for example Patent Document 1).
As shown in FIG. 7, furthermore, there is disclosed a method and plant for producing CDA free from impurities that are unsuitable for semiconductor manufacture such as methane, carbon monoxide, hydrogen, carbon dioxide gas and moisture. Namely, feed air AR is compressed by a compressor 201, compressed air ARp warmed by the corresponding compression heat is further heated by a heater 203 to make compressed heated air ARph, methane, carbon monoxide and hydrogen contained in the feed air are reacted with oxygen in air in a catalyst column 204 to make moisture and carbon dioxide gas, the compressed air ARr left after the reaction is then cooled down to normal temperatures or a temperature less than such temperatures by a cooling facility 205, and the compressed air is thereafter brought into contact with an adsorbent M of an adsorptive purification facility 206 to remove through adsorption impurities such as the converted moisture and carbon dioxide gas contained therein. Thus, clean dry air Ao, in which methane, carbon monoxide, hydrogen, carbon dioxide gas and moisture contained in air have been removed to less than 1 ppm, can be obtained (see: for example Patent Document 2).
As shown in FIG. 8, it is disclosed that exhaust gas discharged from an air separation unit 301 is utilized as regeneration gas for an adsorbent of a CDA producing plant. By utilizing as regeneration gas in a CDA producing plant of the two-column switching system using convention PSA units, exhaust gas of an air separation unit can be effectively used, not influenced by the composition thereof and the use amount of self-produced gas that is used as regeneration gas for the CDA producing plant can be reduced or dispensed. Furthermore, there can be provided a CDA producing method that is cheap and easy in management (see: for example patent Document 3).                (Patent Document 1) Official gazette of Japanese Patent Application Laid-open No. 24,445/2000;        (Patent Document 2) Official gazette of Japanese patent Application Laid-open No. 346,330/2002;        (Patent Document 3) Official gazette of Japanese Patent Application Laid-open No. 326,127/2003.        